Messinian stromatolite-thrombolite associations, Santa Pola, SE Spain: an analogue for the Palaeozoic?

Sedimentology (1997) 44, 893-914

Messinian stromatolite-thrombolite associations, Santa Pola, SE Spain: an analogue for the Palaeozoic? MARK FELDMANN" and JUDITH A. McKENZIEt "Geological Institute, ETH Zentrum, CH-8092 Zurich, Switzerland (E-mail: [email protected]) t Geological Institute, ETH Zentrum, CH-8092 Zurich, Switzerland (E-mail: [email protected])

ABSTRACT

Stromatolite-thrombolite associations are the dominant facies forming large portions of the Santa Pola carbonate platform (SE Spain) during deposition of the Terminal Carbonate Complex (TCC). The TCC, the last period of marine sedimentation in the Western Mediterranean associated with the Messinian Salinity Crisis, comprises a NE-SW trending thrombolite reef with occasionally interlayered stromatolite horizons and a predominantply stromatolite and oolite facies in the back-reef area. The stromatolites are mainly dome shaped, but fine-columnar or wavy-undulose forms can occur. The stromatolites form huge bioherms, extending tens to hundreds of metres. They are finely laminated with alternating layers of dolomicrite and dolomicrospar. The dolomicrite layers appear to be a primary dolomite precipitate, whereas the dolomite crystals in the dolomicrospar layers apparently formed around a meta-stable nuclei which was subsequently dissolved or degraded. The low content of sand-sized particles in the stromatolitic layers indicates formation under low-energy conditions, possibly on a tidal flat. As reported from other areas in the Western Mediterranean, deposition of the TCC at Santa Pola was apparently cyclic, whereby stromatolites generally terminate each depositional cycle. Subtidal Conophyton stromatolites, possibly the only known occurrence younger than Palaeozoic, are, however, found on the reef slope at the base of the first TCC depositional cycle. The dolomitic nature of the unadulterated stromatolitic laminations and the association of stromatolites and thrombolites as platform builders were a common feature in the Early Palaeozoic but are unusual in post-Ordovician carbonate facies. We propose that the conditions during TCC deposition were very restricted, possibly reflecting an environment similar to that of the Early Palaeozoic.

INTRODUCTION

Stromatolites are the principal sedimentary feature of carbonate rocks deposited through the first 3000 myr of geological history. Recognized in the field as laminated, commonly domal, columnar or conoidal structures, stromatolites are generally interpreted as the biosedimentary prod- ucts of sediment trapping, binding, and precipitation by microbial mat communities (Kalkowsky, 1908; Logan et al., 1964; Awramik et a]., 1976). While laminated stromatolites became progressively restricted in abundance and environmental range throughout the late Neoproterozoic and

early Palaeozoic, another type of microbialite concomitantly increased in abundance to become a significant biosedimentary constituent of Cambrian and Ordovician marine carbonates (Kennard & James, 1986). Aitken (1967) intro- duced the term 'thrombolite' for these microbial structures, which are related to stromatolites but without their characteristic lamination. Instead, thrombolites are characterized by a macroscopic clotted fabric which is commonly interpreted to be of cyanobacterial origin (Kennard & James, 1986). Stromatolite-thrombolite associations were significant platform builders in the Early Palaeo- zoic (Aitken, 1967; Kennard & James, 1986). The

0 1997 International Association of Sedimentologists 893

894 M. Feldmann and J. A. McKenzie

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stromatolites commonly were interpreted as reefs growing in low-energy peritidal settings, often in waters of elevated salinity, and the thrombolites as reefs growing under higher energy conditions in the subtidal and normal marine waters (Aitken, 1967; Kennard, 1981). Thrombolites progressively declined in abundance during the early Silurian.

Comparable stromatolite-thrombolite associations of Messinian age were described from Western Mediterranean regions, such as Nijar and Santa Pola (Esteban & Giner, 1977; Esteban, 1979; Bernet-Rollande et al., 1980; Vallbs, 1986; Riding et al., 1991a; Feldmann, 1995; Calvet et al., 1996). These sediments, referred to as the Terminal Carbonate Complex (TCC) (Fig. 1; Esteban, 1979), indicate extraordinary depositional conditions for this interval, the last period of marine sedimentation in the Western Mediterranean associated with the Messinian Salinity Crisis (Hsu et al., 1973). Stromatolites and thrombolites forming carbonate facies, such as these occurring in the Western Mediterranean, are unusual in the post- Ordovician geological record.

This paper describes and interprets the palaeoenvironment of a Messinian example of a stromatolite-thrombolite association near Santa Pola, Spain that forms the platform sediments of the TCC. This study provides a Miocene analogue for Palaeozoic environments where stromatolites and thrombolites were important platform builders. This geologically young example allows environmental conditions to be more confidently evaluated.

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GEOLOGICAL SETTING

La Sierra de Santa Pola is a small carbonate platform of about 20 km2 that forms a peninsula

extending eastward into the Mediterranean. It is located about 15 km south of Alicante and is named after the small village of Santa Pola located on its southern boundary (Fig. 2). The platform emerges east of the Elche plain to form a dolomite hill, 144 m above present sea-level at the lighthouse, Faro de Santa Pola (Fig. 2). In the north, the platform gently dips under a cover of Pliocene to Holocene sediments. The eastern margin of the platform is characterized by a N-S trending barrier reef with a steeply dipping fore- reef slope, of which only the upper part is exposed. The reef wall is constructed almost exclusively by Porites (Esteban, 1977; Esteban & Giner, 1977; Esteban, 1979). Towards the south, the reef front trends NE-SW and reveals mainly thrombolite facies. These two reef fronts comprise two different reefs separated by an erosional surface. The Porites reef matches the descriptions of other Porites reefs reported from the Western Mediterranean (Esteban, 1979; Esteban et al., 1996) and is probably laterally equivalent to the Marine Unit (Fig. 1; Esteban, 1979). The erosional surface represents the major drawdown of the Mediterranean during deposition of the Lower Evaporites (Fig. 1). In contrast, the thrombolitic reef, which is occasionally interlayered with stromatolite horizons, is thought to represent the Terminal Carbonate Complex (Fig. 1). The Marine Unit and the TCC are the Lower and Upper Depositional Units described by Vallbs (1986) and Calvet et a f . (1996). The TCC in the Iagoonal back-reef area comprises mainly oolite and stromatolite facies, which are well exposed and occur at many sites just north of Santa Pola.

STRATIGRAPHY AND SEDIMENTOLOGY

Marine Unit

Fore-reef area

The lowermost sediments which outcrop in the fore-reef area at Santa Pola primarily comprise very fine grained (10-20 pm) dolomite (Profile IX, Fig. 2). This chalky dolomite can be up to 2 m thick (Profile IX, Fig. 3) and contains centimetre- thick, foraminifera-rich limestone layers which are denser than the surrounding dolomite. These layers preserve a variety of planktonic foraminifera, particularly Globorotalia acostaensis, Globorotalia mayeri, and Catapsydrax dissimilis. Nannofossils such as Coccofithus pelagicus, Dictyococcites antarcticus, Reticulofenestra minutula, Reticulofenestra pseudoumbilicus,

0 1997 International Association of Sedimentologists, Sedimentology, 44, 893-914

Messinian stromatolite-thrombolite associations 895

Umbilicosphaera sibogae and Syracosphaera sp. occur attached to foraminifera but were not found in the fine fraction. This bioassemblage indicates sediment deposition in the late Miocene, possibly late Tortonian to early Messinian (Feldmann, 1995). The foraminifera are well sorted, having a size range between 70 and 200pm. In sieved fractions <65 pm and >250pm, no fossil fragments were found. This limited size fraction indicates rapid deposition of the foraminifera1 layers, possibly by storm events when energy differences within the water column winnowed the fine sediment leading to deposition of lag deposits rich in foraminifera. In the upper part of the dolomite, a single 2-cm-thick green-black clay-rich layer occurs containing abundant zircons suggesting a volcanic origin. The chalky dolomite is overlain by a 1 m thick sand-rich cryptalgal dolomite which contains stromatolitic beds (Profile IX, Fig. 3). The cryptalgal dolomite terminates in a con-

glomerate layer, which represents the uppermost horizon of the Marine Unit.

Porites reef

The lower Messinian Marine Unit at the platform edge is mainly represented by a Porites reef (Fig. 1; Feldmann, 1995). The Porites reef consists of several zones which might indicate different growth stages (Esteban, 1977; Esteban & Giner, 1977). A Pinnacle Zone appears above the reef- talus slope with vertical Porites sticks up to more than 1 m high. This zone is followed upwards by a Thicket Zone, characterized by Porites sticks standing vertical to 30" from the vertical. A Transition Zone with subvertical stick Porites and laterally separated convex-downward platy laminar Porites follows, leading to the Reef Crest Zone where vertical sticks disappear and highly irregular laminar coral plates dominate (see also Riding et al., 1991b).

Fig. 2. Location of the study area and geological overview of the units of the carbonate platform, Sierra de Santa Pola, Southeast Spain. The village of Santa Pola marks the southern boundary of the platform. The positions of studied profiles (I to IX) and correlated cross-sections (A-A' and B-B') are indicated. Note the south facing Terminal Carbonate Complex (TCC) thrombolite reef bodies which overgrow the underlying Porites reef (Marine Unit), as seen in the E-W trending canyons. The Pliocene marine sediments onlap the carbonate platform at its northern boundary.


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Back-reef area

The presence of Marine Unit sediments in the back-reef area cannot be determined with certainty. In an old quarry just north of Santa Pola, a clay layer, with similar composition to the clay layer in the fore-reef chalky dolomite (Profile IX, Fig. 3), occurs within a chalky dolomite outcrop (Profile 11, Fig. 4). Unfortunately, the dolomite outcrop of Profile I1 does not contain interbedded foraminifera-rich layers which would allow dating. However, the lithological similarity of this back-reef unit and the chalky dolomites of the fore-reef area permit a reasonable correlation, suggesting that these back-reef area rocks belong to the Marine Unit. The chalky dolomite is overlain by a weakly lithified, 0-5 m thick, bedded dolomite, which might be time- equivalent to the bedded cryptalgal unit of the fore-reef area (Profile IX, Fig. 3). The upper boundary of this bedded unit is sharply separated from an overlying oolite and this boundary is taken as the top of the Marine Unit in the back-reef area.

Lower Evaporites (Erosional Unconformity)

The Lower Evaporites andlor Main Salt are restricted to the more basinal parts of the Mediterranean (Hsii et al., 1977). Thus, marine sedimentation in the Santa Pola area was unlikely since it was probably subaerially exposed during deposition of these evaporites, as indicated by an erosional surface between the Marine Unit and the Terminal Carbonate Complex. The sediments overlying the sand-rich cryptalgal dolomite in the northern part of the platform (Profile IX, Figs 2 & 3) are >4 m thick aeolian sands. There sands are mainly composed of well-rounded and fairly well-sorted peloids (average 200 pm) with abundant angular quartz and calcite grains. The unit has well-developed cross-bedding with high angle foreset dips of about 30", which are indica- tive of terrestrial deposition (McKee & Ward, 1983). From their stratigraphic position, these sandstones could be the terrestrial equivalent of the evaporites deposited in the basins. Thus, the erosional unconformity would be age equivalent to the Lower Evaporites (Fig. 1).

Terminal Carbonate Complex (TCC)

The Terminal Carbonate Complex (TCC), first defined by Esteban (1979) in several Western Mediterranean localities, comprises a distinctive

depositional unit delimited by two regionally important erosional surfaces. The TCC of Santa Pola contains several facies types of which only the most prominent are described here. Esteban (1977), Esteban and Giner (1977), Montenat (1977), Esteban (1979), Bernet-Rollande et al. (1980), Vall6s (1986) and Calvet et al. (1996) give fuller descriptions of the Santa Pola facies.

Porites patch reefs

Porites associations repeatedly appear in the lower part of the TCC and outcrop at several sites (Figs 3 & 4). They occur in patch reefs (Fig. 5), up to 1 2 m across (Calvet et al., 1996) and appear to have grown on peloidal-bioclastic sand (Profile VI, Fig. 4; Profile VII, Fig. 3). Porites occurs as vertical or subvertical sticks, up to 50 cm high. The diameter of the TCC Porites sticks is generally around 1 cm in contrast to the Porites from the lower Messinian reef, which have diameters of 2-3 cm. Their exclusive occurrence in patch- reefs, the smaller stick diameters, and the sandy substrate distinguish them from the lower Messinian Porites. They represent the last recognized occurrence of coral reefs in the area.

Thrombolite reef

The thrombolite reef occurs throughout the front-reef area in the southern part of the peninsula and, also, covers portions of the south- facing Porites reef wall (Fig. 2). The thrombolites are dolomitized and generally have a clotted fabric, distinct from the laminated structures of the stromatolites (Aitken, 1967; Kennard & James, 1986). In thin-section, abundant filamentous structures, possibly algae, about 100 pm across, occur (Fig. 6). In the northern fore-reef area, a dolomite bed with thrombolitic structures and abundant mud lenses overlies the aeolian sand and probably represents the TCC in this region of the platform (Profile IX, Fig. 3). Occasional laths of gypsum, 500 pm long and 100 pm wide, locally fill cavities in the dolomite. The lower part of the thrombolite reef contains abundant carbonate sand and bioclasts; bivalves and gastropods, in the size range of up to several centimetres, are also common. In the upper part, the sand content decreases, fossil fragments become rarer and alter- nations of the thrombolite reef with stromatolitic layers occur. In a few cases, the thrombolite reef contains abundant lime mud lenses of unknown origin. Along the reef front, several bulbous thrombolitic reef bodies overgrew the underlying


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Fig. 5. Lower TCC Porites patch reef. Porites sticks, up to 50-cm length, forming a patch reef a few metres across in the back-reef area at Profile VII. Porites is mainly dissolved away and the moulds are partly infilled by calcite cement. The length of the hammer is 33 cm.

Porites reef in a manner which resembles an onion skin-like structure (Fig. 7). The individual reef bodies, which are frequently separated by later infillings of travertine, indicate several growth stages.

Peloid and bioclast facies

The dominant rock types in the lower part of the TCC are peloidal and bioclastic dolomites, which are distributed over the whole platform. In the central part of the platform (Profile IV, Fig. 3), they form layers up to several metres thick. In the transition zone between the front reef and back reef, they overlay the lower Messinian Porites reef. Peloids and bioclasts are also common com- ponents in the lower portions of the thrombolite reefs, although they disappear with increasing

distance from the front reef toward the back-reef area.

Oolite facies

The oolites, best exposed in a small quarry just north of Santa Pola (Profile 11, Fig. 2), are cross- bedded and contain variable amounts of unidentified monospecific gastropods. Except for these gastropods, fossils are rare. The oolites comprise a prominent depositional horizon in the back-reef area with a thickness > 4 m (Profile 11, Profile I11 & Profile Vb, Fig, 4). The ooids have diameters up to 500pm and are heavily micritized and dolomitized. In some outcrops, the oolites have a foamy mouldic texture because the ooids have been dissolved leaving the surrounding dolomitized micritic cement, which forms a


900 M. Feldmann and J . A. McKenzie

mesh-work pattern. Occasionally, the upper surface of the oolites has been calcretized (Profile 11, Fig. 4).

Stromatolite facies

Extensive bioherms of stromatolites are the most prominent feature of the back-reef sediments of the Santa Pola platform. The transition from the underlying sediments into stromatolites is usually gradual, whereas the upper boundary of each stromatolite unit generally is sharp, indicating cyclic deposition. Stromatolites also occur repeatedly in the front-reef area. Their abundance increases in the upper part of the front-reef section, where they alternate with thrombolitic reef structures. Throughout the platform, stromatolites appear in various forms and have a variety of textures, indicating distinct growth conditions. Based on their forms, the stromatolites of Santa Pola are divided into five distinctive types. Stromatolites rarely occur as individual mounds but appear as bioherms or biostromes extending for several hundred metres laterally. Five types are distinguished:

Fig. 6. Thin-section photomicrograph showing filamentous structures of unidentified algae which are possible framework builders of the thrombolites.


1 Small columnar stromatolites commonly occur within domal and tabular biostromes, which are 30-100cm thick (Fig. 8). These biostromes are generally composed of a series of coalesced, gently convex domes, 0.5-2 m in diameter, and exhibit synoptic reliefs of 10-25 cm. In the back- reef area (Profile 11, Profile I11 & Profile Vb, Fig. 4) they overlie biostromes with wavy-undulose stromatolites and are overlain by thin layers of fine-to-medium peloid and ooid grainstone or flat-pebble conglomerate. In the front-reef area, they overlie domal stromatolites (Profile VI, Fig. 4) or thrombolite reef bodies (Profile I, Fig. 4) and are overlain by thrombolite reefs. Small columnar stromatolites generally range from 2 to 3 cm in diameter, are closely packed with parallel branches, and have crinkled gently convex laminae. They form surfaces which resemble ‘egg carton’ structures and often display desiccation cracks (Fig. 9). 2 Domal stromatolites form gently domed biostromes, up to 2-5m thick and with an average synoptic relief of 5 cm (Fig. 10). The lithologies underlying and overlying these stromatolites in the back-reef area are oolites and wavy-undulose


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Fig. 7. Thromobolite reef body of the TCC (background) overgrowing erosional surface of the early Messinian Porites reef [foreground). The solid line on the drawing indicates the boundary between the two formations. The thrombolite reef bodies overgrew the Porites reef. Each of the layers (separated by the dashed line) could represent a depositional cycle.

stromatolites, respectively (Profile 11, Profile I11 & Profile Vb, Fig. 4). In the front-reef area, they overlie thrombolite bioherms or wavy-tabular strornatolites (Profile VI, Fig. 4). The columns and domes are parallel and unbranched, ranging from 10 to 100 cm in diameter. They are either linked or tightly packed. Occasionally, these stromatolites appear as coalesced domes to form elongated bioherms, indicating the direction of the water currents (Fig. 11). 3 Giant domal stromatolites form gently domed biostromes, up to 70 cm thick. Adjacent lithologies are peloid-ooid grainstones (Profile IV, Fig. 3). Giant domal stromatolites commonly form a

series of coalesced, gently convex domes ranging from 0.5-2 m in diameter (Fig. 12) and exhibit synoptic reliefs of 10-30 cm. 4 Wavy-undulose stromatolites form tabular or sheet-like bodies up to 1 m thick. These biostromes have a synoptic relief of less than 5 cm (Fig. 10). Adjacent lithologies can be small columnar stromatolites, domal stromatolites, thrombolite reef bodies, and rarely peloid-ooid grainstones or flat-pebble conglomerates. The laminations of these stromatolites are planar to slightly wavy. They may be either laterally continuous or discontinuous forming an unconformity-like pattern. Irregular and laminoid



Fig. 8. Small columnar stromatolites forming convex domes which can be up to 2 m in diameter. The length of the hammer is 33 cm.

fenestral fabrics are widespread, and shrinkage or desiccation cracks are locally common. 5 Conophyton stromatolitic structures (Fig. 13) were found at one site in the front-reef slope (Fig. 2) at the base of the first TCC-cycle separating the Porites reef from the thrombolite reef. They have closely spaced vertical columns, up to 5 cm across and 40 cm high, which apparently formed a low-relief bioherm. To our knowledge, these Conophytons are the only representatives of this form that have been recognized in rocks younger than Palaeozoic age.

Examination of the internal structure of the Santa Pola stromatolites, which show distinctive growth forms in different stratigraphic horizons, indicates that their microfabric is not related to their morphologic shape. Within similarly shaped stromatolites, distinct microstructures can occur, whereas similar microstructures can occur within differently shaped stromatolites. The Santa Pola stromatolites generally have laminations consist- ing of couplets of which at least one is dolomicrite enriched in amorphous organic matter. These micritic layers appear dense, bushy, or clotted in thin-section (Fig. 14). They usually contain no detrital grains, although occasionally,

at their base, remnants of micritized detrital grains are found. The micrite layer is generally overlain by a distinctive microspar dolomite layer with crystals ranging from 10 to 40 pm. The crystals have an isodiametric shape showing, more or less, the characteristic rhombohedra1 outlines of dolomite. The crystals contain hollow centres, which sometimes appear as multiples (Fig. 15). At the boundary between the two layers, subspheri- cal fenestral fabrics, up to 300 pm across, frequently occur. The small cavities are occasionally filled with blocky calcite, indicating a later stage diagenesis. The thickness of the dolomicritic and microsparry layers can vary considerably, even within a single thin-section, ranging from 100 pm to 1 mm. The microsparry layer of a couplet often is somewhat thicker than the micritic layer, but there are cases where the opposite is true.

Post TCC (?)

In the back-reef area, stromatolites were found which are separated from the TCC by an angular unconformity (Profile 11,111, and Vb, Fig. 4). These stromatolites have laminations with layers of sparry calcite with crystal sizes of 20-50 pm

0 1997 International Association of Sedirnentologists, Sedimentology, 44, 893-914


Fig. 9. Exposed upper surface of the small columnar stromatolites which form ‘egg-carton’ structures with desiccation cracks. The length of the pencil is about 5 em.

alternating with layers of dolomicrite. Both layers may be up to 1 mm thick but usually measure about 0.5 mm or less. Small cavities or fenestral fabrics are occasionally filled with fibrous calcite, which is orientated perpendicular to the cavity walls. These stromatolite bioherms occur as patches of several tens of square metres. The calcitic layers and the underlying unconformity indicate that these stromatolites may post-date the TCC.

Pliocene

Pliocene sediments are well exposed in the northern part of the Sierra de Santa Pola (Fig. 2; Profile IX, Fig. 3), where they onlap the reef slope and unconformably overlie the TCC. The lower part of these sediments is a yellow calcarenite. The calcarenite is unconformably overlain by a reddish siliciclastic sandstone which is interbedded with coquina shell beds. A more detailed description of these deposits is given by Montenat (1977). In contrast to the deposits of the TCC, these sediments are not dolomitized and their fossil content is well preserved.

DEVELOPMENT OF THE SANTA POLA PLATFORM

Esteban (1979) described Santa Pola as an asymmetric atoll-like platform isolated on the margin of an open shelf. Analogous to other lower Messinian Porites reefs of the Western Mediterranean, the barrier reef of Santa Pola probably developed during the late early Messinian in several stages, possibly as a consequence of a relative sea-level fall (see Haq et a]., 1988). The poor diversity of lower Messinian reefs, built almost exclusively by Porites, probably indicates stress conditions with falling sea level.

When the Mediterranean became isolated, triggering a dramatic sea-level fall during the Messinian Salinity Crisis (Hsii et al., 1977; Muller & Hsii, 1987), deposition of the Lower Evaporites occurred in the deeper basins. The Santa Pola reef was exposed and an erosional surface formed as indicated by calcrete and palaeosols in several outcrops (Profile I & Profile VI, Fig. 4). The Santa Pola Porites reef probably remained subaerially exposed as indicated by the aeolian sand dunes until the intra-Messinian inundation (Fig. 1, Miiller & Hsu, 1987). With rising sea level,

0 1997 International Association of Sedimentologists, Sedirnentology, 44, 893-914


Fig. 10. Well-developed domal stromatolites overlying wavy-undulose stromatolites. The length of the hammer is 33 cm.

deposition of the TCC represents the last interval of marine sedimentation in the Western Mediterranean during the late Messinian. Repeated marine inundations followed by periods of isolation led to cyclic deposition of the TCC at Santa Pola (Section A-A’, Fig. 4). Within this time span, 7 depositional cycles may have occurred (Esteban, 1979; Muller, 1986). However, Calvet et al. (1996) reported three shallowing-upward cycles in the stromatolitic sub-units from Santa Pola. The position of the reef on the shelf might explain the different number of cycles in comparison with the basins. The Santa Pola platform may not have been flooded during each cycle.

With the intra-Messinian inundation, reef growth at Santa Pola resumed but with a different biological composition. The intra-Messinian reef comprises thrombolitic structures and contains variable amounts of gastropods and bivalves. Porites, although present, occurring in the lower TCC in patch reefs, did not play a major role as a reef builder (Profile VI, Fig. 4; Profile VII, Fig. 3). Ooid sands are the dominant sediment type in the back-reef area where they formed shoals several metres thick. The ooids indicate shallow but agitated water in a high energy environment.

Stromatolites were subordinate, but appear pref- erentially at the termination of the cycles, as indicated by a gradual upward growth from the underlying sediment generally with a sharp con- tact to the overlying sediments. They developed in areas with little sand deposition, such as tidal flats or very shallow low-energy environments. They appear to have formed in the zone between low and high tides during relative sea-level high stands and were exposed during the subsequent sea-level drop. The occurrence of Conophyton stromatolites on the upper front reef slope is exceptional (Fig. 13) because they probably formed subtidally (Grotzinger, 1989; Hoffman, 1976) at the beginning of the deposition of the first TCC-cycle prior to the settling and growth of eukaryotic organisms.

During later TCC deposition, peloidal sands and biologic diversity decreased, indicating a change in the water conditions towards a higher salinity. Corals disappeared completely while stromatolites dominated in the back-reef area and thrombolites in the front-reef area. Water circulation probably controlled reef growth at Santa Pola. In contrast to the lower Messinian Porites barrier reef, which trends almost N-S, the TCC



Fig. 11. Coalesced stromatolite domes forming elongated bioherms indicating the direction of the water currents. The direction of the elongation is approximately north-south. The length of the hammer is 30 cm.

thrombolite reef body and elongated stromatolites developed along the southern edge with the reef front trending ENE-WSW (Fig. 2). Reef growth was reduced in the northern part of the platform, where it appears only along east- facing canyons which cut into the Porites reef (Fig. 2).

The formation of the Post TCC stromatolites might have occurred during the short time span of the Lago Mare deposition (Fig. I), a time of primarily lacustrine sedimentation throughout the Mediterranean (Hsu et al., 1977). There stromatolites appear as patches and may have formed in meteoric water fed ponds on the exposed platform that desiccated from time to time. Their alternating calcite and dolomicrite laminae may reflect seasonal changes in salinity, but clear evidence for this assumption is missing.

Stable isotope values indicate the influence of isotopically light meteoric water during calcite (P3C= - 7.90%0, 6l80= - 5.00%0) and dolomicrite (d13C= - 4.00%0, P O = - 1.00%~) precipitation reflecting an environment influenced by fresh water. Later, during the early Pliocene, the Sierra de Santa Pola underwent tectonic uplift and became emergent (Montenat, 1977). The dip of the sediments indicates that uplift at Santa Pola was more or less vertical with a possible tilting of less than 1".

DISCUSSION

Dolomitic stromatolite layers

The dolomicritic layers of the TCC stromatolites consist of microcrystalline dolomite and are


906 M. Feldmann and 1. A. McKenzie

organic carbon-rich, as indicated by a dark, olive green to brown appearance in thin section (Fig. 14). With the exception of the mineralogy and microtexture, these layers show no significant difference to micritic carbonate layers from modern stromatolites, as observed in the Bahamas (Dill et al., 1986; Reid & Browne, 1991; Browne, 1993; Feldmann, 1995; Reid et al., 1995). Based on comparison with modern micritic layers in Bahamian stromatolites, the dolomitic nature of the micritic layers of the Santa Pola stromatolites is interpreted to be a primary or a very early diagenetic feature of primary microbial mat layers. The process leading to micrite-crust formation appears to be rapid, as indicated by the very small crystal sizes, whereby organic matter becomes embedded within the precipitated microcrystals.

Although the micritic dolomite layers in the Santa Pola stromatolites are interpreted as primary features, modern analogues are rare. Lagoa Vermelha, in Brazil, is a modern example where primary dolomite and calcite precipitate in association with microbial mats (Vasconcelos et al., 1995; Vasconcelos & McKenzie, 1997). In this lagoon, the water chemistry is seasonally controlled, with freshwater flooding during the rainy season in winter and intense evaporation

Fig. 12. Giant domal stromatolites interbedded in peloid-ooid sandstones in the quarry to the west of Profile VI.

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with seawater inflow during the dry summer. The precipitate produced in association with organic matter during the wet season is low-magnesium calcite, whereas the evaporative conditions lead to precipitation of primary dolomite during the dry season when the salinity of the lagoon reaches twice that of seawater.

The conditions which produced micritic dolomite in the Santa Pola stromatolites remain speculative to a certain degree. Gebelein and Hoffman (1973) suggest that micritic dolomite layers in stromatolites are secondary. Their assumption is that magnesium ions can be found in high concentrations in organic complexes in microbial mat layers. When the organic matter decomposes, the magnesium is released to form dolomite in the micro-environment of the relict microbial mat layers. However, degradation of organic matter under favourable (anoxic, hypersaline) conditions can lead to primary precipitation of dolomite, as shown in Lagoa Vermelha (Vasconcelos et al., 1995; Vasconcelos & McKenzie, 1997). In modern Bahamian stromatolites the formation of a micritic stromatolite layer may be a process within the micro- environment of a microbial mat, which is associated with boring activity of endolithic microbes


Fig. 13. Conophyton stromatolites (arrows) that separate the overlying TCC thrombolite unit from the early Messinian Porites reef at the base of this first TCC depositional cycle. The length of the hammer is 30 cm.

resulting in dissolution and reprecipitation of carbonate previously produced (Feldmann, 1995; Feldmann, in press). Therefore, the composition of the resultant carbonate precipitate is strongly dependent on the degradation of organic matter and/or the chemical conditions within the microbial mat. Degradation of organic matter by sulphate reducing bacteria can cause the increase of both alkalinity and pH in the pore waters, providing the necessary conditions for dolomite precipitation to occur (Andrews, 1991; Slaughter & Hill, 1991).

The micritic layers of the TCC stromatolites alternate with microsparry dolomite layers, which are considered to be equivalent to the sediment-rich laminae of modern stromatolites. The presence of cylindrical holes, with average

diameters of 10pm in most of the dolomite crystals (Fig. 15) indicates that dolomite growth may have nucleated around minute particles which were subsequently dissolved. These nuclei may have been an earlier meta-stable dolomite precipitate or fine-grained detritus of unknown origin. The growth of the dolomite crystals proceeded outward from the now missing nuclei with the precipitation of discrete layers. Qualitative EDX examination of the layers of an individual crystal shows that the magnesium content tends to increase toward the outer rim (Fig. 16a-d) implying an increase in the stability of each succeeding dolomite layer as stoichio- metric dolomite composition is approached. The microspar layers are porous and permeable and, thus, promote circulation of dolomitizing fluids.

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Fig. 14. Thin-section photomicrograph showing alternating dolomicrospar (lower) and dolomicrite laminae (middle) in a TCC stromatolite. The dolomicritic layer has a bushy top-relief and a relatively sharp bottom line. These features generally allow the determination of the growth direction of a stromatolite. Note the dense, dark appearance of the dolomicritic layer in contrast to the porous, light appearance of the microsparry layer.

Under the assumption that stromatolite formation was coupled with cyclic flooding, which was associated with mat growth and carbonate deposition and precipitation, dolomitization of the microsparry layer could be explained by evaporative dolomitization after flooding (Shinn et al., 1965; Behrens & Land, 1972; McKenzie, 1981). With the decomposition of organic matter under anoxic and evaporative conditions discrete dolomite layers may have formed around nuclei which were subsequently dissolved by circulating fluids.

Cyclicity of stromatolites and thrombolites

Stromatolite-thrombolite associations dominating carbonate facies, such as those at Santa Pola, are unusual in the post-Ordovician. Stromatolites with a wide range of growth forms, such as columns, branching columns, and domes were abundant in the Proterozoic (Hoffman, 1976). These forms represent a variety of environments from subtidal to intertidal and supratidal (Hoffman, 1976). With the increasing occurrence

of thrombolites and the evolutionary explosion of eukaryotes in the Early Palaeozoic (Knoll, 1992; Feldmann & McKenzie, in press) stromatolites, in particular the subtidal forms such as Conophyton, which are well known from early Mid-Proterozoic to Early Palaeozoic (Grotzinger, 1989; Hoffman, 1976), declined or disappeared. Subsequently, the abundance of thrombolites declined with the evolution of reef-forming organisms towards the Middle Ordovician (Kennard & James, 1986).

Considering the various stromatolites and thrombolites which have formed during TCC deposition, we propose that the first TCC cycle, containing Conophyton stromatolites, reflects an environment similar to that of the Proterozoic prior to the evolutionary explosion of the eukaryotes. The subsequent TCC cycles represent an environment similar to that of the Early Palaeozoic when stromatolites were intertidal to supratidal forms, which grew in highly saline environments, and thrombolites subtidal features, which developed in agitated and less highly saline waters.



Fig. 15. SEM photomicrograph showing dolomite crystals in a microsparry layer of a TCC stromatolite. Note the characteristic holes centred in the isodiametric shapes of the rhombohedra1 crystals.

The stratigraphic position of the stromatolites indicates changes in environmental conditions related to the conditions under which the surrounding rocks were deposited. In the front-reef area at Santa Pola, stromatolites are interbedded between thrombolites and commonly terminate depositional cycles (Fig. 17). Calvet et al. (1996) reported three episodes of deposition in the Santa Pola stromatolitic sub-units indicating a cyclic deposition of the TCC probably closely related to an intermittent entrance of marine waters in the Mediterranean. These cycles can be traced from the lagoonal back-reef to the front-reef area and can be separated from each other by sharp contacts, dramatic changes in lithology, or erosional surfaces. In general, with each subsequent cycle up-section in the thrombolite reef, the faunal assemblage becomes poorer and the carbonate sand content of the sediments decreases, whereas stromatolite abundance increases. Analogous depositional cycles within the TCC, which are associated with relative sea- level falls and subsequent rises of at least 30m, were also reported from Las Negras, Spain (Whitesell et al., 1993). Thrombolites are commonly regarded as subtidal forms which develop

in agitated water (Aitken, 1967; Kennard, 1981). If the thrombolites at Santa Pola grew under turbulent high energy conditions acting as a wave breaker, the conditions must have changed during the period of stromatolite formation. Observa- tional studies on modern mats (Feldmann, 1995) demonstrate that very strong wave activity scours and damages stromatolitic structures by removing microbial mats and weakly lithified stromatolite layers. Therefore, the formation of a stromatolite primarily requires a laminar water flow, where scouring is low such as in tidal currents, or a relative decrease in high energy conditions. Such conditions would occur with a shallowing caused by a sea-level drop or during initial flooding of a platform during a sea-level rise.

The stromatolites formed in the front-reef area generally appear to have grown upward from the underlying reef body with a sharp boundary to the overlying sediments. They repeatedly appear as elongated bioherms (Fig. 11). These features suggest that they formed during falling relative sea-level under agitated water conditions, possibly produced by tidal currents. Beside these physical conditions, the chemical conditions may have changed as indicated by the different



000 1 a

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B

I 1' 2 3' f. ZOO sec Znergy (ke;

C D

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Fig. 16. (A) SEM photomicrograph showing a cross-section through a dolomite crystal from a microsparry layer in a TCC stromatolite. Note the growth of the crystal by precipitation of discrete layers. The x markers (b, c, and d) indicate the sites of EDX examination. (B)-(D) EDX-plots. qualitative chemical analyses, showing that the Mg content tends to increase from site B to D. Site B, appears to be a low magnesium calcite; Site C, a high magnesium calcite; and site D, a dolomite. This zonation indicates secondary dolomitization.

bio-assemblages. The thrombolites always contain a variety of organisms, such as bivalves and gastropods, even if their diversity decreased with time during cyclic TCC sedimentation. In contrast, the stromatolites are fossil-poor. An expla- nation for this change in biological abundance may be falling relative sea-level leading to a

restricted environment. The thrombolites may have formed in a subtidal environment a few metres deep and under more or less normal marine conditions. With falling sea-level, the reef environment changed from subtidal to peritidal, and, during low tides, high evaporation led to hypersaline conditions. Under these conditions,



Increasing energy

Back-reef area ! Front-reef area strornatolites thrombolites

Fig. 17. Schematic illustration showing stromatolite and thrombolite growth relative to cyclic sea-level changes during the Intra-Messinian flooding event. (A) Relative high sea-level stand with thrombolite growth in the subtidal zone at the front reef, oolite deposition in the deeper parts of the back-reef area, and stromatolite growth in the intertidal zone of the back-reef area. (B) Relative low sea-level stand promoting thrombolite growth in the basinward migrated subtidal zone at the front reef and stromatolite growth in the intertidal zone on both the previously deposited ooid sands and thrombolites. (C) Relative high sea-level stand with thrombolites growing over the preceding stromatolites in the subtidal zone at the front reef, stromatolite growth in the intertidal zone of the back-reef area, and sand deposition in the deeper water of the back-reef area.


912

the reef builders of the thrombolites disappeared, whereas the salinity resistant microbes thrived to form stromatolites. With continuing sea-level fall, the stromatolites were exposed and possibly partly eroded. With subsequent flooding during a sea-level rise, stromatolites could form again in the front-reef area, but the rising sea-level led to a change from laminar to turbulent energy conditions, which caused destruction of the weakly lithified stromatolites and again supported thrombolite growth.

In contrast to the front-reef area where stromatolites stratigraphically indicate termination of depositional cycles associated with relative sea-level falls, stromatolites in the lagoonal area represent entire depositional cycles (Fig. 17). Here, they occur as tabular or distinc- tively shaped domes forming giant bioherms covering extended areas of a few hundred square metres. Their morphology and extent imply growth under calm conditions, where the stromatolite-forming microbial mats could develop undisturbed. The salinity was probably high due to evaporation, as indicated by the absence of skeletal organisms. Each of the stromatolite shapes may represent growth in different water depths. For example, with rising sea-level, the area around Profile I1 (Fig. 4) was flooded (at the beginning only during high tides), and the surface was colonized by microbes and tabular stratiform to wavy-undulose forms developed on these tidal flats. These tabular stromatolites could not keep pace with continuing sea-level rise and domal forms developed in the new subtidal environment. This change in morphology permitted faster growth allowing the mat forming cyanobacteria to receive sufficient light for maximum growth. With the subsequent sea-level fall, renewed peritidal conditions dominated, causing a return to tabular forms. With supratidal conditions, disruption of the extended microbial mats by periodic desiccation caused the formation of small adjacent microbial mat patches. Under these conditions, small columnar stromatolites may have formed as indicated by the abundant desiccation cracks. Slightly elongated stromatolites probably formed in channel depressions, where stronger laminar water flow controlled their growth direction.

M. Feldmann and J. A. McKenzie

CONCLUSIONS

1 The Santa Pola platform is composed of two reef types which are separated by a major

erosional surface: (a) an early Messinian N-S striking Porites reef which formed prior to the deposition of the Terminal Carbonate Complex and (b) a NE-SW trending thrombolite reef, which is accompanied by lagoonal oolite and stromatolite facies, that developed cyclically during deposition of the TCC. 2 Thrombolites formed under subtidal high- energy conditions. Back-reef stromatolites, having distinct forms, grew in intertidal to supratidal low-energy environments. 3 Conophyton stromatolites, to our knowledge the only representatives of this form that have been recognized in rocks younger than Palaeo- zoic, occur at the front-reef slope and probably formed subtidally prior to the colonization of eukaryotes during the first TCC cycle. 4 The stromatolites have alternating layers of dolomicrite and dolomicrospar. The dolomicrite is interpreted to have formed primarily associated with the bacterial degradation of organic matter. The dolomicrospar is probably a product of evaporative dolomitization. 5 The Santa Pola stromatolite-thrombolite associations, forming large portions of the platform sediments, possibly reflect an environment similar to Palaeozoic environments where stromatolites and thrombolites were important platform builders.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support provided by the Geological Institute, ETH Zurich. Special thanks to our colleagues at Swiss Federal Institute of Technology, Zurich; to M. CortBs, E. Jacobs, M. Pika-Biolzi, and K. von Salis for the determination of the micro- and nannofossils, to U. Gerber and D. Grujic for their photo- graphic and SEM support and to H. P. Schielly Jr. for his help in the field. The careful reviews of M. Esteban and M. Pedley significantly improved the early version of this manuscript.

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Manuscript received 12 July 1996; revision accepted 20 December 1996.

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